SUMMER 2021

68 THE DISTRIBUTOR’S

68 THE DISTRIBUTOR’S LINK SPIROL INTERNATIONAL CORP. 30 Rock Avenue, Danielson, CT 06239 TEL 1-860-774-8571 FAX 1-860-774-2048 EMAIL info@spirol.com WEB www.spirol.com FASTENING TECHNIQUES FOR PLASTIC HOUSINGS by Jeff Greenwood, Product Sales Engineer Plastics have become more and more common in manufacturing, and there is still plenty of room for growth. As such, Engineers must ensure that new plastic products meet performance requirements while considering component costs and assembly cycle time. While plastic materials provide many advantages, they can introduce manufacturing challenges during the assembly process that lead to inefficiencies, scrapped parts, and added costs. This article addresses the advantages and disadvantages of the following fastening methods used to assemble plastic components: solid pins, adhesives, screws, bolts, and snap-fit joints. Serviceable vs Permanent Before selecting the fastening method, designers must decide if their product needs to be serviceable. For example, products like fire detectors, television remotes, and key fobs must be designed so that consumers can disassemble the product to replace batteries. Conversely, products like hair dryers and single-use medical devices are typically not designed to be disassembled or serviced. Furthermore, some designers will specifically design components to be tamper resistant so that consumers cannot easily disassemble the product (i.e. electronics). Serviceable Screws, bolts, and snap-fit joints (i.e. plastic buckles) are the most common fastening methods used in serviceable products. Bolts and screws provide a clamp load that compresses the components within an assembly. This is accomplished when the bolt/screw is torqued beyond the point at which the head makes contact with the mating component. Bolts and screws provide superior retention compared to snap-fit joints, which rely on the strength of the plastic component itself. FIGURE 1 - BACK OF A POCKET CALCULATOR Snap-fit joints provide retention by interlocking multiple components together. Snap-fit joints can be assembled quickly and they simplify the overall assembly process because there are fewer parts on the bill of materials. Snap-fit joints are often preferred when minimal retention is needed in a plastic product. For example shown in Figure 1, most pocket calculators use snap-fit joints for the battery cover so that consumers can easily change the batteries. Permanent Solid pins, adhesives, screws, bolts, and snapfit joints are all commonly used to fasten permanent assemblies. While designers may not specifically require a new product design to be serviceable, many products still use serviceable fasteners like screws for convenience and operator familiarity. For example shown in Figure 1, most pocket calculators use screws to connect the plastic housing together even though consumers do not need to pull the assembly apart during the useful life of the product. TECHNICAL ARTICLE CONTINUED ON PAGE 138